This invention relates to compositions for use in fracturing underground formations such as gas wells, particularly the fracturing of low permeability formations using such compositions.
The fracturing process involves the injecting of a treating fluid down to the wellbore and into the productive formation at a sufficient rate and pressure so that the formation rock fractures from the induced stresses. A proppant added into the fluid prevents closure of the fracture when hydraulic pressures are released, thereby leaving a conductive flow channel from the wellbore deep into the rock matrix.
For several years liquid carbon dioxide has been added to fracturing fluids to assist in post-treatment clean up. It was applied with oil or water based treating fluids at various ratios. Higher concentrations of liquid carbon dioxide in the fracturing fluid have been proven beneficial, leading to a reduction in the volume of treating fluids. The latest development in the fracturing process uses highly energized liquid carbon dioxide as the sole proppant carrier. The application of pure liquid carbon dioxide eliminates most of the formation damage normally associated with convention fracturing fluids and enables a very rapid clean up and evaluation of the well following the stimulation. However, when pure liquid carbon dioxide is applied and a sufficient fracture width not produced, a significant decline in productivity can be observed within several months after the stimulation.
Density and viscosity are the most important characteristics of the fracturing fluid. Both density and viscosity affect the carrying capacity of the fluid for sand, while viscosity is primarily responsible for the final fracture shape and profile. A wide range of additives are used to enhance the rheological and chemical properties of the oil, water or alcohol base treating fluids. Liquid carbon dioxide has a very low viscosity and its physical, chemical and thermodynamic properties over wide range of temperatures occurring during the fracturing processes limit the choice of additives considerably.
Several concepts were considered for the thickening of liquid carbon dioxide. For fracturing purposes an ideal situation would have the viscosity of thickened carbon dioxide decrease at bottom hole conditions without leaving solid or liquid residue in the formation. One way to achieve this was to have a novel composition using the concept of copolymerization of liquid carbon dioxide and propylene oxide as disclosed in U.S. Pat. No. 4,374,545. However, this system requires precise preparation, timing and handling of chemicals.
It is the object of the present invention to overcome the disadvantages of the prior art and provide a simpler and more effective fracturing process utilizing the conventional fracturing equipment now in use.